69 rats underwent temporary or permanent ligation of the left coronary artery and were studied by the injection of colloidal carbon following fixation-perfusion. 10 rats were using the same protocol and served as controls. Localized myocardial ischemia was accompanied by microvascular changes which produced capillary obstruction when blood flow was reestablished. This phenomenon of "no-reflow" was characterized by the presence of large non perfused areas seen after brief periods of ischemia (15 min). These areas were increased when the period of ischemia was lengthened. After 30 to 60 min of interruption of blood flow the non perfused area extended over the major portion of the ischemic area. During reperfusion the "no-reflow" phenomenon displayed during the first hour showing a transitory improvement in capillary perfusion which was soon followed by a progressive reexpansion of the non injected zones. After 24 hours of reperfusion, the latter zones were identical in their extent to those cases showing tissue necrosis following permanent ischemia. This "no-reflow" phenomenon appears to play a role in the evolution of the reperfused ischemic area by excluding certain areas from the benefits of reperfusion. The most probable factors involved in this process are: increased blood viscosity, endothelial changes peri-capillary edema and the contractile state of the myocardium. The incidence of these microvascular changes, using various methods of myocardial preservation during open heart surgery operations, as well as the present attempts directed towards metabolic therapy of myocardial anoxia, are under investigation.